Hitherto, a polyvinyl alcohol-based film has been produced by dissolving a polyvinyl alcohol-based resin in a solvent such as water to prepare a stock solution, subsequently forming a film by a solution-casting method (a casting method) and drying the film using a metal heating roll or the like. The polyvinyl alcohol-based film thus obtained has been utilized in a large number of applications as a film excellent in transparency, and a polarizing film is exemplified as one of useful applications thereof. Such a polarizing film has been used as an elemental constituent element of liquid crystal displays and in recent years, its use has been extended to equipments for which high definition and high reliability are required.
Under such circumstances, as brightness and fineness of the screen of a liquid crystal television or the like are heightened, there has been required a polarizing film more excellent in optical properties than conventional ones.
For such requirement, as polarizing films obtained as a result of improvement of a polyvinyl alcohol-based resin, there have been, for example, proposed a polarizing film using, as a base material, a uniaxially stretched film comprising a polyvinyl alcohol-based polymer wherein an amount of a 1,2-glycol bond directly bonded to the main chain is 1.8% by mol or more (see e.g., Patent Document 1), a polarizing film formed of a polyvinyl alcohol film comprising a vinyl alcohol-based polymer containing 0.01 to 20% by mol of a cationic group-containing unit and 0.5 to 24% by mol of an α-olefin unit having 4 or less carbon atoms (see e.g., Patent Document 2), and the like. Moreover, as a means of enhancing polarization performance in the vicinity of a wavelength of 400 to 500 nm, there has been proposed washing with an at least 0.8% aqueous potassium iodide solution in a washing step that is a production step of polarizing films (see e.g., Patent Document 3).    Patent Document 1: JP-A-8-136728    Patent Document 2: JP-A-2003-248123    Patent Document 3: JP-A-2004-341503